The present invention is directed to a necking machine for can bodies.
In the production of can bodies, especially of so-called two-piece cans with a one-piece blank for beverages and cosmetics, etc., a plurality of processing steps must be carried out on the open end of the can blank. On one hand, the margin (the edge) that has received an irregular shape in the deep-drawing or elongation process must be cut, typically on a trimming machine. Furthermore, the can blanks are often necked at the open end, in order to form a flared tube end or a thread with a flared tube end. Covers or lids whose diameters typically have, in the area connecting to the can body or can blank, a smaller diameter than the base of the can are often placed on the necked, open end. These processing steps are carried out on so-called necking machines. These machines comprise an annular work-piece holder that is arranged on the machine housing of the necking machine. On a corresponding annular tool holder, a plurality of tools are arranged for carrying out the individual processing steps. The tool carrier sits on a cylindrical tube that can be shifted in the axial direction in the machine housing and can be moved in the axial direction by a crank drive. Before each work stroke, the work pieces arranged on the work-piece holder are rotated by a specified angle, so that for each stroke, a subsequent tool comes into contact with the can bodies set on the work-piece carrier.
Currently, the number of cycles of necking machines equals about 200 cycles per minute.
The processing time is the time during which the processing steps, such as milling, rolling threads, rolling beads, or forming flared tube ends, are carried out on the work pieces with turning tools. These processing steps take place in the last phase of each work stroke, i.e., approximately in the last 25 mm before the dead center. For the processing of the work pieces, there remains about 0.043 seconds. If the number of cycles were increased by 25%, then the processing time would be reduced accordingly. This would lead to a reduction in quality.
Through the design of the crank drive and displacement of the axis of rotation of the crank drive from the axis of the advancing piston, it is possible to change individual phases of the advancing movement slightly compared to the law of motion of the planar crank drive. However, such changes are always associated with accompanying, unavoidable effects on other areas of the path of motion. In addition, the accelerations then have unequal profiles at both dead centers. This means that the machine must be designed for a permissible maximum value. During a cycle, however, this is reached shortly only once. Depending on the ratio between the stroke length and connecting-rod length of the machine, the achieved maximum value of the acceleration in the half of the movement with the lower loads lies between about 75% (shorter stroke) and 50% (longer stroke) of the maximum value of the total motion.
Another possibility for influencing the law of motion is the direct drive by a servomotor, which, however, would represent an inefficient method due to the large masses being moved (>1000 kg) and the resulting high driving energy. A linear drive could be constructed as a directly acting linear motor or as a direct drive of the crankshaft. In the second case, the accuracy of the machine is given by the crank mechanics. Only the path of motion must be reset each time. In the first case, both magnitudes for each stroke must be newly set by the electronics, which would mean a higher risk of failure.